Colouration in English Cockers?

Hi all, I am after some advice. I am seeking a blue roan and tan cocker puppy. The breeder I have found has a litter of 3 day old pups; all blue roan but none with signs of tan as of yet. The breeder said in the previous litter there were a couple of roan and tan pups, but the tan wasn’t visible until later.
Can pups be born without even a hint of tan and still develop it? Would there be subtle signs of tan by 2 weeks of age? Thanks in advance!